Atherosclerosis is a vascular disease characterized by a modification of the walls of blood-carrying vessels. Such modifications, when they occur at discrete locations or pockets of diseased vessels, are referred to as plaques. Certain types of plaques are associated with acute events such as stroke or myocardial infarction. These plaques are referred to as “vulnerable plaques.” A vulnerable plaque typically includes a lipid-containing pool separated from the blood by a thin fibrous cap. In response to elevated intraluminal pressure or vasospasm, the fibrous cap can become disrupted, exposing the contents of the plaque to the flowing blood. The resulting thrombus can lead to ischemia or to the shedding of emboli.
One method of locating vulnerable plaque is to peer through the arterial wall with infrared light. To do so, one inserts a catheter through the lumen of the artery. The catheter includes a delivery fiber for illuminating a spot on the arterial wall with infrared light. A portion of the light penetrates the blood and arterial wall, scatters off structures within the wall and re-enters the lumen. This re-entrant light can be collected by a collection fiber within the catheter and subjected to spectroscopic analysis. This type of diffuse reflectance spectroscopy can be used to determine chemical composition of arterial tissue, including key constituents believed to be associated with vulnerable plaque such as lipid content.
Another method of locating vulnerable plaque is to use optical coherence tomography (OCT) to image the arterial tissue surrounding the lumen. To use this method, one also inserts a catheter through the lumen of the artery. The catheter includes a fiber that transports light having a limited coherence length through imaging optics to the arterial wall. The backscattered light couples back into the fiber towards an interferometer. The interferometer provides a cross-correlation signal that is used to map the shape of the arterial tissue. This map of the morphology of the arterial wall can be used to detect the fibrous cap and other structural characteristics associated with vulnerable plaque.